1. Field of the Invention
The invention is related to the field of communication networks and, in particular, to providing for online charging for sessions of dual mode devices that are transferred between network domains (e.g., a session transferred (seamlessly handed over) from an IMS network to a circuit-switched network).
2. Statement of the Problem
One type of communication network gaining popularity is an IP Multimedia Subsystem (IMS) network. As set forth in the 3rd Generation Partnership Project (3GPP), IMS provides a common core network having a network architecture that allows for various types of access networks. The access network between a communication device and the IMS network may be a cellular network (e.g., CDMA or GSM), a WLAN (e.g., WiFi or WiMAX), an Ethernet network, or another type of wireless or wireline access network. The IMS architecture is initially defined by the 3GPP to provide multimedia services to communication devices over an Internet Protocol (IP) network, as IP networks have become the most cost savings bearer network to transmit video, voice, and data. Service providers are accepting this architecture in next generation network evolution.
For a typical session (or call) within an IMS network, user equipment of an IMS subscriber initiates the session through an access network, such as a CDMA network, a GSM network, an IP network, a WiFi network, a WiMAX network, etc, by transmitting the appropriate signaling messages (i.e., SIP messages). The access network then routes the signaling messages to the IMS network. If the access network does not use the same signaling protocols as the IMS network (e.g., SIP), then the access network may route the signaling messages to the IMS network through an appropriate gateway. A serving-call session control function (S-CSCF) in the IMS network receives the signaling messages and attempts to establish the session in the appropriate manner. When the session is established, the S-CSCF may also contact one or more application servers (AS) in the IMS network to provide services for the session, such as voicemail, call forwarding, etc.
To provide online charging (prepaid charging) for the session, each of the IMS network elements (e.g., S-CSCF and AS) handling the session may generate charging messages typically in Diameter Ro protocol. For instance, a network element transmits a Diameter Credit Control Request (CCR[Initial]) message to an Online Charging System (OCS) in the IMS network at an initial triggering event to request a credit quota for the session. Periodically during the session, the network element transmits Diameter CCR[Update] messages to the OCS to request additional credit quotas if needed. At an ending triggering event, the network element transmits a Diameter CCR[Final] message to the OCS to return any unused credits. Each of the charging messages includes an IMS Charging Identifier (ICID) that is assigned to the session so that the charging messages may be correlated in the OCS.
Some service providers allow for dual mode service. Dual mode service allows a communication device to communicate with different types of network domains that utilize different protocols. Typical network domains include a circuit-switched domain, a packet-switched domain, a WLAN domain, and an IMS domain. As an example, dual mode service may allow a communication device to communicate with a circuit-switched network, such as a CDMA network or a GSM network, and also communicate with an IMS network through the appropriate access network. Communication devices that are able to receive a dual mode service are referred to as dual mode devices or dual mode handsets.
When a dual mode device initiates or accepts a session over a first network domain, such as an IMS network, the serving network element in the first network domain determines that the dual mode device has dual mode capabilities and contacts a handover application server (also referred to as a Voice Call Continuity (VCC) application server) presumably in the IMS network. For example, in an IMS network, an S-CSCF contacts the handover application server through SIP or another protocol to indicate the dual mode capabilities of the device. The network element in the first network domain then establishes the session with a destination and a first sub-session is established between the dual mode device and the destination. In response to the first sub-session being established, a first charging identifier is assigned to the session in the first network domain. The first charging identifier is assigned by one of the network elements in the first network domain. The network elements that serve the session generate online charging requests that include the first charging identifier, and transmit the online charging requests to the OCS. The OCS then determines a charging rate for the first sub-session, and generates credit quotas for the network elements serving the first sub-session as is known for online charging.
If the dual mode device moves from the first network domain to a second network domain, such as a circuit-switched network, then the dual mode device transmits another session initiation request to the circuit-switched network. When a dual mode device transfers from one network domain to another network domain, this is often referred to as a “handover”. An MSC in the circuit-switched network receives the new session initiation request, and forwards the message to the handover application server. The handover application server operates to establish a second sub-session between the dual mode device and the destination through the second network domain. The handover application server establishes the second session leg so that the session is not interrupted in the handover to the second network domain. The end user keeps the session when actually the session is seamlessly handed over from the first network domain to the second network domain. The end user may not notice the handover during the session. The first sub-session is also torn down.
In response to the second sub-session being established, a second charging identifier is assigned to the session in the second network domain. The network elements in the second network domain that serve the session generate online charging requests that include the second charging identifier, and transmit the online charging requests to the OCS. The OCS then determines a charging rate for the second sub-session, and generates credit quotas for the network elements serving the second sub-session as is known for online charging.
One problem with dual mode services is that there is no effective way to perform online charging for a session when there was a handover from one network domain to another network domain. When there are multiple handovers during one session, a different charging identifier is generated in each network domain. Thus, the OCS is not able to correlate the online charging requests for the same session that was transferred from one network domain to another, and thus cannot correctly determine a charging rate for the session. For example, when a user initiates a session over an IMS network using a dual mode device, the S-CSCF in the IMS network and handover application server trigger online charging requests to the OCS. The OCS grants credit quotas for the first sub-session using the charging rate defined for the IMS network. After 30 minutes, assume that the user walks out of his/her home with the dual mode device and the session continues over a circuit-switched network (e.g., a CDMA network). The MSC in the circuit-switched network triggers an ANSI ORREQ or ANLYZD operation to the OCS. The OCS considers this as a new session, and grants a new credit quota to the MSC with rate calculated from time=0. This calculation is incorrect when the user has a service plan with a stepped tariff, as the OCS should start the rate calculation from time=30 minutes. Unfortunately, without context information for the session, the OCS cannot correlate the ANSI ORREQ or ANLYZD request with the correct starting time and determine an accurate charging rate for the sub-session over the circuit-switched network.